1. Field of the Invention
The present invention relates to modified polytetrafluoroethylene (hereinafter referred to as "PTFE") and a process for preparing the same. More particularly, the present invention relates to modified PTFE which is suitable for producing a porous article by low pressure compression molding and has good mechanical properties, particularly creep resistance and a process for preparing such modified PTFE.
2. Description of the Related Art
A porous PTFE article is produced by, for example, by drawing an unsintered PTFE film at a temperature lower than the melting point of PTFE to make it porous and then sintering the drawn PTFE film while maintaining the drawn state at a temperature higher than the melting point of PTFE. By such a method, only a porous PTFE film having a thickness of from several microns to 100 microns is produced, but a porous article having a desired shape cannot be prepared.
Then, it is proposed to prepare a porous PTFE article by compression molding unsintered PTFE powder under low pressure followed by sintering at a temperature higher than the melting point of PTFE. However, the density of PTFE powder increases considerably during sintering, so that it is difficult to produce a porous article having a large porosity.
It is also proposed to prepare a porous PTFE article by sintering unsintered PTFE powder at a temperature higher than the melting point of PTFE, grinding the sintered PTFE powder, press molding the ground powder followed by sintering at a temperature higher than the melting point of PTFE (cf. Japanese Patent Kokai Publication No. 66730/1986). By this method, the steps for preparing the sintered PTFE powder are complicated.
Further, it is proposed to produce a porous PTFE article by molding PTFE powder together with an organic material which has no compatibility with PTFE and can be removed in a subsequent step and removing such material from the molded PTFE article to make it porous. In this method, the porous PTFE article may contain the unremoved material. Therefore, not only is the production step, but also the inspection step becomes complicated.
PTFE is known to be a resin having excellent properties such as heat resistance and chemical resistance, but is poor in creep resistance.
PTFE is commercially produced by polymerizing tetrafluoroethylene by suspension polymerization, emulsion polymerization and the like. All the conventionally utilized polymerization techniques use radial polymerization initiators. No method has been proposed to prepare PTFE having stable properties and with good reproducibility, in the absence of a polymerization initiator.